Satellite communication systems rely on transponders within a satellite to receive the signal sent from a ground station, shift the frequency and filter and amplify it before it is sent back to the earth to the receive station(s). Each transponder has a fixed bandwidth. For example, many satellites have a transponder spacing of 40 MHz with a bandwidth of around 36 MHz. Conventional transponders receive weak signals, amplify the signal strength, translate it to the downlink frequency, filter unwanted sidebands and then amplify the signal again to send the amplified signal to the receiver site. A side effect of using filters and amplifiers is the introduction of amplitude and group delay variation versus frequency, which limits the usable bandwidth. These effects happen in the uplink equipment as well, but usually to a lesser degree.
FIG. 1 is a schematic representation of a conventional satellite communication system including a transmitter system (left), a satellite repeater 150 and a receiver system (right). The transmitter system receives a digital data input 100 after it passes through digital baseband processing 110. The signal is directed to modulator 120 which modulates the digital data onto a carrier. Modulated data signal is then converted to the appropriate frequency and filtered by upconverter 130. The data signal is then directed to High Power Amplifier (“HPA”) 140 to amplify the communication signal prior to transmitting the signal from the transponder system antenna 148 to the satellite repeater 150.
The signal is received at the satellite repeater 150 as an uplink signal received by antenna 152. In a typical satellite repeater 150, the uplink signal is processed through LNA 154, down converter 155, filter 156, amplifier 158 and filter 159 before its transmission through antenna 153 to the receiver system antenna 172 as a downlink signal.
The downlink signal received through antenna 172 is direct downlinked to a LNB converter 174 which amplifies the signal but inherently adds thermal noise. The data signal is then input to demodulator 178 at L-band. The demodulator recovers the originally-transmitted data to provide digital data output 180. Alternatively, the receiving system 170 could comprise a low noise amplifier (“LNA”), radio frequency (“RF”) to intermediate frequency (“IF”) down converter and a demodulator that accepts the IF for demodulation.
Any part of the signal transfer chain from transponder system to satellite repeater to receiver system that imparts a change in amplitude or group delay versus frequency will cause a degradation of the signal. These changes cause a degradation in performance of the demodulation process, and thus, a less reliable system. The largest contributors to the degradation of the signal are caused by the group delay of the upconverter and filter 130 and the satellite repeater 150 which adds group delays at each of its filters 156 and 159. Various parts of the transmitter system and satellite repeater 150 conventionally introduce significant amplitude distortion as well.
Most commonly, the amplitude and phase delay distortion is minimized through the bandwidth of the signal being kept narrow enough to occupy only a limited portion of the available transponder bandwidth where the group delay is sufficiently small to only minimally affect the signal. Another common approach is to place an analog equalizer in the ground station uplink that is tuned to compensate for this group delay characteristic. Analog equalizers comprise several sections of all-pass filters that cannot remove the excess delay at the edges of the transponder bandwidth, but rather add additional delay in the middle. This is accomplished in a piecewise method by manually tuning all the sections while monitoring the downlink with very expensive test equipment. To tune the various sections is an art rather than a science. It is impossible to completely equalize the channel with this device. Significant residual group delay or amplitude flatness issues will remain and are subject to the typical drift of analog components.